This invention relates generally to devices for metering liquids. More particularly, this invention relates to metering apparatus for metering repeated equal constant volumes of fluid samples.
In counting particles or other materials such as blood cells suspended in a fluid medium, it will be appreciated that it is necessary that a precise degree of control be carried out in the flow of the liquid suspension being examined. That is, the degree of control of the flow of the material affects the accuracy of the determination. In blood cell counters, it is necessary to accurately and precisely control and measure the flow of the sample suspension past the scanning point of the detecting system so that a constant volume of the sample suspension is metered through the detecting system. A high order of repeatability of flow of the sample suspension through the detecting system, especially of different suspensions, is particularly desirable in the interest of efficiency and economy. It will be appreciated, further, that changes in the metered volume of a sample suspension due to outside influences such as ordinary temperature changes must be accommodated.
Thus, it is the object of this invention to provide a novel fluid metering apparatus which enables a more precise and accurate metering of a pre-determined and constant volume of the fluid to be tested as it passes the scanning point of a detecting system than has been realized in the past.
The above is achieved, in accordance with this invention, by the use of low permeability elastomeric diaphragms in the vent and sample volume connections of the manometer of the invention in order to isolate the mercury contained therein from contamination, and to reduce the necessity of having to clean the instrument. Moreover, a one-way valve is utilized in the air vent stack of the manometer which valve closes off the flow of mercury almost immediately after the mercury meniscus passes the "stop" point in the measurement cycle. The purpose of the one-way valve is to reduce to a minimum air flow into the counting aperture, once the counting period has ceased. The valve causes immediate cessation of flow once the measuring function has taken place. The valve may be a ball valve comprised of a material inert to mercury such as, for example, polytetrafluoroethylene.
Also provided in accordance with this invention is the use of a slide valve for closing off the vent stack and isolating the mercury from the vent diaphragm cavity during shipping. This has the effect of restricting the movement of the mercury during any jolting going on during shipment or in the storage and handling of a manometer.
Additionally, the start and stop points in the mercury flow path of the manometer of the invention are simple wire tips extending into the mercury flow passage for engaging the axis of the mercury meniscus. This provides a much more precise initiation and cessation of the counting or metering period. In this connection, the electrode contacts may be, for example, molybdenum contact wires having an amalgam of mercury at the contact ends thereof in order to produce a mercury-to-mercury contact. Such a contact is self-renewing, thus making the instrument, in accordance herewith much more precise and long lasting.
A further important feature of the invention is a preset fixed volume sample or metering tube which is positioned horizontally so as to eliminate the need for calibration. That is, because of the preset volume of the tube, no calibration is necessary. In this connection, the substantially horizontal sample tube is positioned deliberately with a slight tilt between the start and stop count electrodes so that the very minor change in height of the medium when it passes through the conduit balances the change in pressure due to the diaphragm displacements at either end of the instrument.
This has the effect of satisfying a range of volume compliance values which is the result of a change in the vacuum applied to the aperture system during the counting period. This changing pressure multiplied by the system volume compliance, yields a volume change of the conduits and their contents, (including air) which means that the volume measured by the manometer is not identical to the volume of a sample drawn through the aperture of the counting instrument being metered. Thus, there is an error in volume measurement. The slight tilting to accommodate this volume compliance has the effect of overcoming this phenomenon.
Other objects and advantages of this invention will be apparent from the following description, the accompanying drawings, and the appended claims.